The present invention relates to an electron beam-graftable compound. More particularly, the present invention relates to a compound which can be grafted to a hydrophobic surface under electron beam radiation without homopolymerizing, thereby altering the surface characteristics of the hydrophobic surface.
Polymers are used widely throughout the world to make a variety of products which include blown and cast films, extruded sheets, injection molded articles, foams, blow molded articles, extruded pipe, monofilaments, and nonwoven webs. Some of such polymers, such as polyolefins, are naturally hydrophobic, and for many uses this property is either a positive attribute or at least not a disadvantage.
There are a number of uses for polyolefins, however, where their hydrophobic nature either limits their usefulness or requires some effort to modify the surface characteristics of the shaped articles made therefrom. By way of example, polyolefins are used to manufacture nonwoven webs which are employed in the construction of such disposable absorbent articles as diapers, feminine care products, incontinence products, wipes, and the like. Frequently, such nonwoven webs need to be wettable, especially when the webs are to be used as, by way of example only, diaper liners, feminine pad transfer layers, tampon covers, and industrial wipes.
The wettability of normally hydrophobic nonwoven webs can be obtained by spraying or coating the web with a surfactant solution during or after its formation. The web then must be dried, and the surfactant which remains on the web is removed upon exposure of the web to aqueous media; thus, the imparted hydrophilicity is transient and cannot withstand multiple wettings.
Alternatively, a surfactant can be included in the polymer which is to be melt-processed, as disclosed in U.S. Pat. Nos. 3,973,068 and 4,070,218 to Weber. In that case, however, the surfactant must be forced to the surface of the fibers from which the web is formed. This typically is done by heating the web on a series of steam-heated rolls or "hot cans". This process, called "blooming", is expensive and still has the disadvantage of ready removal of the surfactant by aqueous media. Moreover, the surfactant has a tendency to migrate back into the fiber which adversely affects shelf life, particularly at high storage temperatures. In addition, it is not possible to incorporate in the polymer levels of surfactant much above 1 percent by weight because of severe processability problems; surfactant levels at the surface appear to be limited to a maximum of about 0.33 percent by weight. Most importantly, the blooming process results in web shrinkage in the cross-machine direction and a significant loss in web tensile strength.
Chemically modifying the surfaces of fibers and nonwoven webs has been achieved by radiation-induced polymerization and also by chemical reactions at the fiber surfaces. For example, the irradiation of polymerizable materials can result in the formation of block or graft interpolymers due to cross-linking (abstraction of .alpha.-methylenic hydrogen atoms) or chain scission between carbon-carbon bonds; see, e.g., P. Alexander et al., Proc. Royal Soc. (London), 1954, A223, 892.
The efficiency of actual grafting, however, depends on the relative susceptibilities of the monomer and polymer to the ionizing energy. For example, if the number of radicals per 100 electron volts (eV) of absorbed ionizing energy for the monomer is much greater than that of the polymer, homopolymerization will predominate with little or no grafting. See F. A. Bovey, "The Effects of Ionizing Radiation on Natural and Synthetic High Polymers Review Series," 1958, 1, Interscience, N.Y.
Typical methods of irradiation-induced grafting employed in the surface modification of polymers are as follows:
(a) the simultaneous irradiation of polymer in the presence of excess monomer;
(b) the irradiation of polymer prodipped in monomer;
(c) the pre-irradiation of polymer (in the absence of oxygen), followed by exposure to monomer; and
(d) the pre-irradiation of the polymer in air to form peroxides which subsequently decompose in the presence of monomer.
A number of vinylic monomers have been grafted onto polypropylene fibers. See, by way of illustration, Japanese Patent No. 23810, 1961; U.S. Pat. No. 2,999,056 to Tanner; and British Patent No. 839,483. For example, glycidyl methacrylate or acrylate was grafted by irradiation onto polypropylene fibers containing benzophenone. Polyethylene film has been grafted with acrylic acid by both simultaneous irradiation and pre-irradiation techniques.
The use of synthetic fibers in apparel fabrics has led to heightened interest in the aspects of hydrophilic surfaces that relate to comfort, such as softness, wicking, absorption of water, and transport of moisture through and from the body. Much work, for example, has been reported on making polyester fabrics (woven or non-woven) as comfortable as cotton (S. G. Hall et at., Textile Chemist and Colorist, 1977, 9, 20). To achieve this objective, methods were developed to chemically polymerize several hydrophilic monomers on polyester fabrics to give cross-linked finishes having various ionic and nonionic groups. The finishes are swellable and give fabrics the ability to hold large amounts of water without a significant restriction on wicking. Nylon fabrics also can be made hydrophilic by means of the same procedure.
In addition, numerous papers have been published on how to make polyolefins (such as polyethylene and polypropylene) hydrophilic using irradiation procedures. See, for example, Z. Foltynoxicz et al., Macromolecules, 1985, 18, 1394; U.S. Pat. No. 4,100,309 to Micklus et al.; and A. Baszkin et al., J. Colloid Interfce Science, 1973, 43, 190. As of now, most of the developments have centered on coating or impregnating the hydrophilic species onto the substrates. These coated or impregnated surfaces, however, typically are not permanent and often are removed if organic or aqueous solvents come in contact with the treated substrates.
In summary, most coatings for electron beam work involve simple vinyl-or acrylate-based chemistry. On exposure to electron beam radiation (or other radical-generating source), the primary reaction is one of homopolymerization. The resulting homopolymer is simply trapped in the void spaces of the nonwoven web. This is deleterious for the following reasons:
(a) there is a stiffening of the fabric, or loss of drape;
(b) There is little or no modification of the fiber surfaces;
(c) The fabric has the surface properties of both the homopolymer and the original, typically hydrophobic fabric; and
(d) there typically is a loss of or reduction in fabric porosity.
Several radiation technologies other than electron beams have been used to chemically modify the surfaces of polymeric materials to achieve such hydrophilicity, but with varying disadvantages. Ultraviolet and cobalt-60 gamma radiation are mainly applied to substrates immersed in aqueous solutions or other solvent systems. Radio-frequency plasma radiation treatments may be carried out with gaseous materials, but at low pressures.
Electron beam radiation, however, is the radiation treatment of choice. Electron beam radiation can be applied as part of a semi-dry process without solvents, and its fast reaction time (10.sup.-2 sec) makes it suitable for an on-line continuous process. Interaction of electron-beam radiation with a polymeric material generates free radicals on the polymer surface. These free radicals will attack vinylic species present on the surface, making possible the attachment of surface-modifying moleties to a polymeric substrate. However, the efficiency of this grafting reaction is dependent upon the tendency of the monomer to homopolymerize, or react with itself instead of the substrate.
A new class of compounds has been discovered for use with an electron beam process which cannot significantly homopolymerize because of the steric hindrance designed into the molecular structure. However, the compounds still undergo rapid radical formation with the concomitant grafting of the compounds onto the surfaces of fibers and nonwoven webs. Thus, significant and permanent changes in the surface characteristics of the fibers and nonwoven webs are possible without the foregoing disadvantages typically associated with radiation-induced surface modification techniques.